13 Apr
2010
13 Apr
'10
12:11 p.m.
On Mon, 12 Apr 2010, Tony Duell wrote:
I traced things a bit further. The 24V line from the power connector runs
directly to the NO contact of a relay. The leg that runs through the
scorched resistor ends up at a TO220 transister. I'll try to get more
detail on its downstream side.
I'm wondering if the resistor might be there to limit inrush current?
Maybe the relay closes at servo lock and feeds power directly to the
driver transistors at that point?
I've had a look at the Memorex 112 manual on Bitsavers. If your drive is
similar, you might bave big problems.
The relay contact is part of a 4-pole relay used to turn on the spindle
motor (the other 3 poles connect braking resistors ot the windings) The
10 Ohm (5W) resistor is the fred to the collector of a power transistor,
whcih is the pass transistor for a simple 12V regulator. The emitter of
that transistor is the output of the 12V regualtor and feeds a lot of
analuge circuitry.
My guess is that the transsitor is OK (but it would be worth checking) and
that you have something downstream of it that is drawing exceessive
current. It might be a shorted decoupling capacitor, it might be an IC.
And to make things worse there seem to be some custom analogue ICs in
this drive.
First test that I would do is to measure the voltages (wrt ground) on the
3 leads of that transsitor.
At
initial power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
It would be worth knowing what that resistor feeds. The obvious things to
need the 24V rail are the spindle motor and positioner (which could be a
stepper nmotor in an old/small drive like this, or it might be a voice
coil). I would also suspect that the 24V line is regualted down to 12V
for some of the analogue circuitry. The heads are not stuck. If I tip the drive, I can
watch them slide
across the platter from their own weight. Should have mentioned before
that this thing has a translucent plastic top :-).
RIght..The Model 112 is a stapepr motor positioned, BTW. That is what I'd
expect in a low capacity, but large platter drive like this. The trackls
would be suufficiently wide that a stepper motor would be fine. You need
a voice coil positioner taking a servo signal from the disk if you have a
high tranck density, of course.
-tony